Bearing steel must be highly pure in terms of non-metallic inclusions.
Non-metallic inclusions have a significant impact on the mechanical properties of bearing steel, particularly its contact fatigue performance, and thus affect the service life of bearing components.
From a metallurgical perspective, producing bearing steel with low levels of non-metallic inclusions and high purity has long been an important area of research.
Nonmetallic inclusions in bearing steel originate from raw materials, deoxidation products, slag, and refractory materials, as well as secondary oxides formed during tapping and casting.
Inclusions found in bearing steel can be classified into two major categories based on their origin: extrinsic inclusions and endogenous inclusions.
Exogenous inclusions include slag, refractory materials, and mold-protective slag that remain in the steel after being failed to be removed, as well as secondary oxides formed in the molten steel.
Exogenous inclusions tend to have larger particle sizes and irregular shapes—often star-shaped or polygonal—with relatively complex compositions and structures. They are unrelated to the composition of the molten steel and pose a significant threat to the properties of bearing steel; therefore, their presence is generally prohibited by industry standards.
Therefore, various measures should be taken, such as improving the quality of refractory materials used in ladles and the casting system, as well as the quality of their installation, and ensuring a clean and hygienic environment for the steel outlet channel and the casting system.
Endogenous inclusions are formed during smelting and casting by the following reactions: When deoxidizers are added to molten steel during tapping and ladle refining, the products generated by the deoxidation reaction are called primary oxides;
As the molten steel cools from the pouring temperature to the solidification temperature, since deoxidation reactions are exothermic, the equilibrium shifts as the temperature decreases, causing the deoxidation reaction to resume; the products of this reaction are referred to as secondary inclusions;
As the molten steel solidifies, decarburization reactions continue in the two-phase region between the molten steel and the crystals, even extending to cooling below the solidus temperature. The re-precipitation of inclusions caused by phase transformations results in products known as tertiary products.
The smelting, casting, and solidification of bearing steel constitute a complex physicochemical process. The various compounds formed are fine inclusions that, prior to solidification, fail to rise to the surface of the ingot and become embedded in the steel, or are dissolved in the steel at high temperatures. As the temperature decreases during solidification, the solubility decreases, causing these compounds to precipitate from the molten steel and exist as inclusions.
Sulfides, oxides, and point-shaped non-deforming inclusions in bearing steel all fall under the category of endogenous inclusions.
Bearing steel is the steel grade with the most stringent quality requirements among specialty steels. Impurities have a significant—and even catastrophic—impact on steel properties; therefore, only by thoroughly understanding the sources and formation processes of impurities in steel can they be effectively mitigated, thereby ensuring the performance and quality of the steel.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.